It will be appreciated by those skilled in the art that in order to use an integrated circuit chip, it is necessary to make electrical connection to the integrated circuit incorporated within the chip. These connections are made through contact pads located at one main face (hereinafter called the interconnect face) of the chip. The contact pads are connected, e.g. using bond wires, to conductive strips of a substrate member. The package of the integrated circuit chip and the substrate is mounted on an etched circuit board (ECB), the conductive strips of the substrate member being connected at their distal ends to circuit runs of the ECB. In this manner, the integrated circuit is connected to other components mounted on the ECB.
It is frequently desired that several chips be mounted on a single substrate. However, when multiple chips are mounted on a common substrate, the need for re-work in the event that a chip proves defective may arise. Thus, if a device comprises several chips, one of which is defective, it may be desired to remove the defective chip and replace it with a good chip rather than scrap the entire device. However, removal of a defective chip and reattachment of a good chip is time consuming in the case of a device employing wire bonds, because each bond has to be broken and then reestablished.
The disadvantage of using wire bonding is avoided in the case of the "flip-chip" technique. By this method, the substrate, e.g. of ceramic material, carries conductive strips that terminate at the perimeter of the "footprint" of the chip in a pattern of connection points corresponding to the pattern of contact pads on the chip, and the chip is placed on the substrate with its interconnect face down and its contact pads in direct physical contact with the connection points. The bond pads or the connection points carry solder preforms, and by applying heat and pressure through the chip a direct metallurgical bond is established between the chip's contact pads and the substrate's connection points. However, the flip-chip technique is subject to the disadvantage that in the event of a mismatch between the coefficients of thermal expansion of the chip and of the substrate, the temperature range within which the packaged integrated circuit can be used without the reliability of the circuit being impaired owing to differential thermal expansion is quite small.
A third packaging method involves use of an epoxy adhesive to bond the chips to a substrate, which may be of essentially rigid ceramic material or it may be flexible. However, removal of a defective chip may cause damage to the substrate.